Before discussing this topic, it is important to clarify that the risk of fire in a thermal oil installation is extremely low, as the safety measures and protocols for thermal fluid boilers have been optimised for decades. Both the manufacturers and the users of these systems understand the importance of proper design, use and maintenance in ensuring the safety of personnel and installations.
Here at Pirobloc, we are aware that safety is the most important aspect of any thermal oil installation, and we are committed to offering our customers a highly efficient, safe, reliable product.
Causes of fire in industrial hot oil heating systems
The main causes of fire that can occur in a thermal fluid installation are the following:
Leaks usually occur in installations that are poorly designed or inadequately maintained. They are the result of failures in components, such as seals, gaskets or hoses.
When a large volume of thermal fluid leaks and the hot fluid comes into contact with an ignition source, if the temperature is above the ignition temperature, this can be a direct cause of fire. The most common ignition sources are any bearings, electrical panels, pumps or motors which are located below the leak. If the ignition source is part of the defective component, or if it is the source of the leak, it may lead to a major fire.
Small leaks on the flanges can also cause safety problems. This type of leak can be minimized by using appropriate flanges and gaskets, made from graphite or fibre-reinforced Teflon material, depending on service temperatures.
This is a highly unlikely occurrence, as it requires the combination of two events that we have never seen at any installation: an oil leak and a poor choice of insulation material.
Insulation fires occur when heat transfer fluid leaks and seeps into porous insulation materials such as calcium silicate, fibreglass wool or mineral wool. The open structure of porous insulation allows the heat transfer fluid to move away from the leakage and spread throughout the insulation. Spontaneous ignition can cause the fluid to suddenly become exposed to the air. For example, if the protective cover is punctured, or if the liner is removed for a maintenance operation.
Self-ignition of the insulating material can also occur under two conditions:
- When the molecules of the insulating material are rearranged in such a way as to create a new compound, which catches fire due to the temperature and oxygen levels of the insulating material.
- When there is a rapid increase in the oxygen level.
For this reason, the selection of the right insulation material can significantly reduce the risk of fire.
The most effective precaution that can be taken against insulation fires is to identify all potential leakage points, and to use closed cell foam glass insulation at these points, as this prevents the fluid from spreading to the rest of the insulation. Another option is to have no insulation at these potential leakage points.
The areas where the highest risk of fluid leakage typically exists are valves, Y-strainers, pressure taps and all types of connections.
Undetected loss of flow
Loss of flow occurs when a series of faults in the installation lead to an interruption in the thermal fluid circuit. Although this is an extremely rare occurrence, fires can be caused by a loss of flow when combined with a failure of the safety devices.
- The initial failure usually occurs in one of the equipment’s components, such as the pump motor, a clogged filter or a coupling. This interrupts the normal flow of thermal oil to the boiler.
- Thermal fluid boilers incorporate safety devices which are designed to shut down the heat source if there is not enough oil flowing through the boiler, or if there is excessive outlet temperature. This is where the second system failure takes place, as these systems can fail as a result of soiling, wear and tear or poor location.
As we have already mentioned, nowadays, the chances that a fire will occur in a thermal oil installation are almost nil. However, Pirobloc has high-safety, tri-redundant SIL systems that can guarantee zero fire risk.
In spite of this, should these circumstances occur while the heater is operational, the temperature of the piping and shell will rise rapidly, and they may well rupture due to thermal stress because, as the burner continues to supply energy to the stagnant fluid, the temperature rises rapidly beyond the AIT. If this happens, fluid that is leaking outside the boiler through a crack will ignite as soon as it is exposed to the air, if its temperature is above the flash point.
If the equipment and piping remain intact, the vaporized fluid can be discharged through the relief and/or vehicular valve to the expansion tank, where it will be discharged into a collection tank.
To avoid this type of fire, both the low oil flow shutdown system and the rest of the safety devices must be located on the burner safety interlock.
Serious fires caused by coil tubes that are made from unsuitable materials are very rare, but precautions must be taken to ensure that they will not occur. The recommended material for the coils is ASME ASTM A106 Gr. B Sch. 40 piping. The nominal thickness of 3″ piping is 5.5 mm.
Cracks are formed as a result of excessive thermal cycling, or around overheated spots that develop in the piping due to internal soiling or impact from flames.
This situation is not dangerous if the boiler is in operation, as any thermal fluid that leaks into the combustion chamber through the resulting cracks will burn as fuel while the boiler is operating.
However, this situation is extremely dangerous when the burner is switched off, as fluid will continue to leak into the combustion chamber due to the pressure from the expansion tank and the upper piping. This will cause a large pool of thermal oil to form inside the combustion chamber, which will explode into flames the next time the boiler is lit.
The condition of the thermal fluid may be another potential risk. It is advisable to establish a maintenance program that includes periodic analysis of the thermal fluid. Oxidation occurs if the expansion tank remains hot (over 140°F) during normal operation, and is open to the air. The reaction of the hot fluid with the air forms tars and sludges, which can coat surfaces and reduce heat transfer. These deposits may create hot spots in the heater, which can eventually cause cracks. Oxidation, which can also be detected by routine fluid analysis, may be prevented by keeping the expansion tank cool (below 40°F) and preventing air from getting in.
How to asses the fire risk of a thermal fluid
Experts highlight three parameters that can help to assess the fire risk of an installation:
- The flash point, which is the temperature at which the vapour of a fluid will temporarily burst into flames when exposed to a source of ignition.
- The ignition point, which is the temperature at which a temporary flame will continue to burn the available fuel source. The ignition point is usually between 4.5°C and 37.5°C above the flash point. Both the flash point and the ignition point are dependent on the presence of an ignition source, such as a spark.
- The autoignition temperature marks the point at which no ignition source is necessary for a fire to occur.
While the parameters described above are important for understanding the fire risk for thermal fluids, plant managers and engineers must take other aspects into account when designing the installation, as very few fires have originated from heat transfer fluid. In most cases, the overheated thermal fluid is not the reason for the fire, but rather the equipment and systems that transport it. As discussed above, this can often be attributed to poor choice of insulation materials, loss of flow, cracked heater tubes, leaks that are not contained, or poor equipment and circuitry.
Fire prevention measures
Although the risk of fire is very low, there are a number of measures that can be taken in order to drastically reduce the risk of fire in a thermal fluid installation. Most of these measures relate to circuit design and maintenance. In addition, the main suppliers of thermal oil and heat transfer fluid systems have safety guidelines relating to the design and installation of thermal oil systems, which must also be adopted.
Furthermore, the correct operation and maintenance of thermal fluid systems reduces the risk of fire. Maintenance should include daily and weekly inspections, looking for signs of smoke at potential leak points, especially at the valves, flanges, welds, instrument ports and threaded fittings. Proper maintenance goes a long way to ensuring the safety of the installation.
Below, we detail a series of specific safety measures that every thermal fluid circuit should provide for:
- Thermal insulation is one of the main components in fire protection. The use of closed cell foam glass is recommended, as it will prevent any liquid leaks from spreading to the rest of the insulating material. This waterproof material should be installed around any component that has the potential to leak.
- Porous insulation, such as mineral wool, fibreglass and calcite, should be used with great care in hot oil systems, if at all. It can be used for straight lengths of piping, but there must be sufficient space with no insulating material around possible leakage points.
- Pump seals should be replaced as soon as there is a leak.
- Flanges should not be insulated. Metal covers can be installed, if necessary, for the protection of personnel.
- To avoid incidents resulting from loss of flow, low flow shutdown should be included on the burner safety interlock
- Include a low flow alarm in the expansion tank that will notify an operator and shut down the system if the oil level in the circuit is below normal levels.
- Centralise the main equipment and install containment berms in those areas, in order to contain spills and isolate potential incident locations where possible.
- Drip pans should be kept free of liquid.
- To avoid excessive thermal cycling of the boiler tube bundle, the manufacturer should reduce the output of large heaters.
- Allow expansion joints and flexible hoses to move along the length of their shafts, never sideways.
- Carry out proper maintenance of lubrication systems for rotating joints, and supply the appropriate lubricating oils for these systems.
- Valves should be installed with their stems facing sideways, so that any leakage will run down the stem and away from the piping. If the stems are slightly lower than horizontal, they will allow any leaking liquid to drip out of the valve body.
- Use the recommended stem packing for globe, ball or plug valves, when they are used with thermal fluid.
- Install isolation and bleed valves in the piping for each piece of equipment, so that maintenance can be performed without draining the entire system.
- The overflow tank should be a vented closed head tank with a drain valve. It should be located far away from the exit doors and the boiler control panel. Never vent the overflow tank inside the boiler room.
- Any area where there is a risk of leakage should be adequately ventilated, to prevent the accumulation of flammable vapours. If smoke is being produced around a leak, this is a good sign, as it indicates that vapours are not building up.
- Hydraulic systems and lines should not be installed near a boiler, as they have the potential to spray fluid across long distances.
- The floor area around pumps, skids and boilers should have a dyke that will contain any major spills.
The importance of thermal insulation in industrial heating
Thermal insulation solutions are used to cover hot oil piping and equipment that is operating at temperatures above ambient temperature, contributing to the safety of operators and facilities. On the one hand, these solutions prevent flammable liquids from being absorbed, thus ensuring maximum fire safety. And on the other hand, they limit heat transfer and surface temperature, for the protection of personnel.
The safest type of insulation for a thermal oil installation is closed cell insulation that is made from cellular glass, such as FOAMGLAS®. This material is created in such a way that the air pockets in the cellular glass material do not merge. This means that any fluid which gets into the insulating material will remain separate, near the leakage point, thus preventing any auto-ignition that could lead to fires.
The material is usually perforated with drainage holes, so that any thermal oil that accumulates on the insulation can drain easily, thus significantly reducing the fire hazard. It is important that the drainage is not located directly on an ignition source, as this could also cause a fire.
FOAMGLAS® cellular glass insulation is the safest insulation for a thermal oil system. However, it is also the most expensive option, as both the material and its installation require a greater investment than other alternatives.
There are other types of insulating materials which may include compressed particles, such as perlite and calcium silicate, or mineral fibres, but this type of insulation is not recommended for points in the installation where there is a potential danger of leakage.
One option for optimising thermal insulation costs is to combine the use of cellular glass insulation with other, cheaper options. In this way, the closed cell insulator would be used at any points in the circuit where there is a medium or high risk of leakage (valves, Y-shaped filters, pressure taps and all types of connections). The more economical insulator would then be applied anywhere that there is a low risk of leakage, such as horizontal piping sections.
Once again, we would like to emphasise the importance of proper design and constant maintenance of the installation in order to prevent possible risks. Whichever insulation option is chosen, checks should be carried out frequently when the thermal oil system is operational. In this way, it will be possible to check whether any thermal fluid is leaking into the insulation material, and act accordingly.
If any smoke or odour is detected in relation to the thermal fluid, a thorough check should be carried out in order to verify that there is no leak in the circuit. As already mentioned, when you have a suspected leak, one preventive option is to perforate the liner with drainage holes, so that the leaked liquid can be easily drained.
If a darkening in the colour of the insulating material or liner is observed, it is highly likely that there is a liquid leak. In this case, any insulation that thermal fluid has seeped into must be removed very carefully, because any spontaneous increase in the oxygen level may cause the insulation to auto-ignite.